1. Technical Field
The present invention relates generally to semiconductor device fabrication, and more particularly, to methods of forming a transistor, and the transistor so formed, including a channel of a high mobility semiconductor material.
2. Related Art
Semiconductor devices have continued to decrease in size. One challenge facing continued size reduction is developing high performance devices with smaller gate lengths. One approach to increase performance with smaller gate lengths is to increase the carrier mobility, i.e., electron and/or hole, in the channel. Although it is possible to obtain higher carrier mobilities with strained silicon, much higher mobilities can be achieved by using a different semiconductor material in the channel other than silicon. For example, hole mobility in silicon-germanium (SiGe) and germanium (Ge) is known to be much higher than in silicon. However, using a bulk material other than silicon presents huge integration and device design challenges. For example, defects due to the relaxation of SiGe are a large problem. In addition, diffusion enhancement of n-type dopants in SiGe and Ge makes it very challenging to obtain advantageous ultra-shallow junctions. Moreover, most of the process steps such as silicidation and surface cleaning are currently unsatisfactory. As a result, it continues to be easier and cheaper to use silicon as a bulk material and use the high mobility materials only in the channel.
In order to achieve a SiGe or Ge channel transistor structure, SiGe or Ge can be epitaxially grown on silicon to include a strain. Unfortunately, however, exposure of these materials to required high temperature steps, e.g., for implant damage annealing, can relax the strain, causing a multitude of defects to form. These defects can make the devices inoperative and/or can cause large yield losses. Moreover, high temperature steps can cause too much n-type dopant diffusion through these materials and can cause the source/drain regions to short, or can cause significant short channel effects. Strained materials such as SiGe exhibit better critical thickness and stability at lower temperatures. For example, a higher Ge concentration SiGe can be stable on silicon if it is grown and maintained at lower temperatures. Unfortunately, no process currently exists to generate this structure and not expose the materials to high temperature steps.
Another approach to increase mobility in silicon is to use super steep retrograde wells (SSRW). The term “retrograde well” indicates that the well is formed using an approach in which the highest concentration of dopant (implanted) in the well is located at a certain distance from the surface, which makes the gate electrode smaller and less susceptible to punch-through. The term “super-steep” indicates that the transition from the lower concentration of dopant to the higher concentration is fairly abrupt, i.e., a dopant profile has a super-steep attribute at the transition. SSRW devices are advantageous because they can have undoped silicon in the channel because short channel effects can be controlled by the steep well. A challenge relative to SSRW devices, however, is controlling dopant diffusion into the undoped silicon channel during the high temperature steps. The undesired diffusion can happen either from the steep wells or from source/drain extensions.
In view of the foregoing, there is a need in the art for a process that does not suffer from the problems of the related art.